The pointing direction or line of sight of an airborne camera that is fixed to the body of an aircraft moves about as the aircraft maneuvers. For example, as the aircraft rolls, the camera rolls up or down depending on which side of the aircraft it is mounted. If the camera is inertially stabilized, rather than fixed to the body of the aircraft, its line of sight will remain in a fixed direction relative to the body of the aircraft as the aircraft maneuvers. When the camera is inertially stabilized, its field of view sweeps across a scene at a velocity equal to the overflight velocity of the aircraft. For example, if the camera is pointing directly down with its scan aligned with the body of the aircraft and the aircraft is traveling at 360 mph and if the camera's field of view is one mile, then an object entering the field of view will exit the field of view in 10 seconds.
Inertial stabilization is typically effected by mounting an airborne camera within a gimbal system, and driving the gimbals based on the roll, pitch, and heading of the aircraft. Such an approach can deliver high-quality stabilized pointing, but aircraft overflight velocity is not removed. Operator inputs are used to command changes in line of sight to compensate for overflight velocity. Such systems are the subject of U.S. Pat. No. 5,897,223; U.S. Pat. No. 3,638,502; U.S. Pat. No. 4,989,466; U.S. Pat. No. 4,643,539; and U.S. Pat. No. 5,184,521. FIG. 1 illustrates a prior technique that does not correct for overflight velocity of the aircraft over the target. In such prior art, the line of sight of the camera is inertially stabilized. In this example, as the aircraft 100 circles 101, the camera's line of sight sweeps out a circle 102. This stabilization sweeps the imaged area across the ground at a velocity equal to aircraft overflight velocity. To compensate for the overflight velocity, an operator would manually need to keep the camera pointing at the target.
Automatic compensation for aircraft overflight velocity causes the line of sight to remain fixed upon a selected position relative to the ground. If the gimbal system performs this compensation automatically, then the operator is relieved of this burden.